It is common to provide LEDs, such as blue-emitting GaN LEDs, on a substrate and apply a yellow phosphor (e.g., YAG phosphor) over the LEDs to create white light. Any color lamp may be formed using LEDs and phosphors. The phosphor may be applied over each LED die, or the phosphor may be applied as a continuous layer over a layer of LEDs. In all such cases, the phosphor is located between the LEDs and the light exit surface of the lamp.
Phosphor is relatively expensive, and a minimum of phosphor should be used to accomplish its wavelength-conversion function to create the desired output color. A phosphor layer may be formed of phosphor powder in a transparent binder, or the phosphor powder may be directly applied over the LED surface, such as by electrophoresis.
When the phosphor is located between the LEDs and the light exit window of the lamp, some LED light typically leaks through the phosphor, and the combination of the LED light and the phosphor light creates the desired color. The phosphor layer must be of a thickness and density so that there is a precise percentage of the LED light that gets converted and a precise percentage that leaks through.
For lamps using a continuous layer of phosphor over a layer of LEDs, the phosphor area directly over a blue LED will appear blue due to the high concentration of blue light directly beneath it. This is referred to as a blue hot spot. The light will appear more yellowish, assuming a yellow phosphor is used, for areas further from the LED due to the lower concentration of blue light. This creates non-uniform color across the lamp.
In lamps where each LED is coated with a thin layer of phosphor, the lamp will have bright spots unless diffusing optics are used. Such optics attenuate the light, reducing efficiency.
In some cases, such as when the layer of LEDs is formed by printing many thousands of microscopic LEDs as a monolayer on a substrate, it is impractical to coat each LED with a layer of phosphor, and only a continuous layer of phosphor can be used.
What is needed is an LED lamp design that reduces the required amount of phosphor and improves color uniformity across the lamp.